The invention relates to torsionally rigid flexible shaft couplings of the type exemplified in German published patent documents DE 197 09 950 B4 and DE 197 09 951 C2, each owned by the assignee of the present invention.
In the conventional embodiments of these couplings, what is sought is as high a torque to be transmitted in combination with a sufficient ability of accepting misalignment (i.e., angular axial and radial misalignment) of the various parts of the coupling relative to each other.
In order to provide for a sufficient misalignment in known couplings, sufficient clearances must be present between plate packs 5 and the hub 1 or sleeve 8 of the coupling (see especially FIGS. 2 and 3). In this respect, an important measure is the distance 10 between the planar hub face and the face of the plate pack, which corresponds to the flange clearance of the flanged sleeve engaging the hub.
In the prior art, the ratio of the flange clearance 10 in FIG. 2 to the outer diameter 16 in FIG. 4 is 1:40 at most, or smaller.
The surfaces of the coupling plates may be roughened as described in DE 197 09 951 C2 so that a relatively low bias will suffice to frictionally transmit comparatively high torques to the flanged sleeves and, thence, to the hub. If the geometry of the flanged sleeves corresponds to DE 197 09 950 B4, an absolute clearance between the plates, the hub and the adjoining sleeve will be safely maintained at normal power densities.
After laser equipment and other laser test gear have greatly enhanced the users' abilities of ascertaining the radial and angular misalignments to be leveled out by the coupling, it is not the main concern any longer to compensate for high misalignments of torsionally rigid flexible shaft couplings but more so their power density, with poser density being defined as a torque as high as possible transmissible with an outer diameter as small as possible.
The problem underlying the present invention is to substantially raise the torque transmissible frictionally and, thus, without any play, by means of a torsionally rigid flexible friction-type coupling, This problem is solved by the present invention.
The proposed design accepts a reduction of the misalignment to 50% to 75% of the otherwise conventional values. By reducing the misalignment to approx. 50% to 75%, the required clearances between hub 1 and plate pack 6, or between plate pack 5 and sleeve 8, can be reduced so as to enable higher stiffness within plate pack 5 to be provided.
As shown in the prior art FIGS. 1-3, lever arm 11 between the center of plate pack 6 and the planar end face of hub 1 creates by way of the existing circumferential force 12 in FIG. 3 a bending torque 15 in FIG. 3 acting upon flanged sleeve 6 and the threaded bolt 7 installed in the through-bore of that sleeve, In the state-of-the art situation shown in FIGS. 1-3, very high bending torques may, in extreme cases, cause the flanged sleeve to lift off or disengage on one side, causing inadmissible increases of the bending torque component acting on the bolt—this can lead to potential fracture of the bolt or the sleeve under the dynamic loads acting on the bolt.
At a coefficient of friction equal to 0.1 between flanged sleeve 6 and hub 1, the torque acting between the sleeve and the hub is transmitted via combined frictional and positive (i.e. shape-locked) engagement. In the prior art of DE 197 09 951, coefficients of friction of 0.3 may be obtained by partially roughing the plates in the bolted area of threaded connection. As a result, the torque frictionally transmissible in the plate pack is higher by a factor of 3 than that transmissible between the flanged sleeve and the hub. For compensating the difference, the existing positive engagement is used, although this will result in corresponding loads on, and thus deformation in, the flanged sleeves. Very high torques may cause considerable micromovements between the flanged sleeve and the hub and consequently result in frictional corrosion as well as slip between the input and output sides.
In accordance with the invention, the flanged sleeves are coated in the areas of radiussed recess 6e, thus substantially increasing the frictionally transmissible component of the torque. The surface roughening may be obtained by abrasive blasting (e.g. sand blasting) or by coating (e.g. Durni Disp SiC or Ekagrip). In the process, the flanged sleeve is roughened, with the different hardnesses of the materials to be combined being important. The flanged sleeve must be harder than the surfaces of engagement.
Further the flanged sleeves are made of high=strength heat-treated steel (such as 42CrMo4 or 51CrV4 with a yield strength of at least 750N/mm2) so that existing loads will be sustained.
In accordance with the invention, and in contrast to the state of the art in DE 197 09 950 B4, the axial length of the flanged sleeve is reduced as far as possible so as to keep the distance between the planar hub face and the plate pack face (flange clearance 10 in FIG. 2) as small as possible. As a result, lever arm 11 in FIG. 2, as well as the bending torque 15 in FIG. 3 acting on flanged sleeves 6, will be markedly reduced.
Further the plates in the plate pack used in accordance with the invention are selected to be thicker, as shown in FIGS. 4 and 5, so as to reduce the buckling load of the pressure-loaded segments of a plate pack.
The aforesaid measures result in an increase of the transmissible torque by a factor of 1.8 to 2 over the current state of the art. Another advantage is that the unchanged bolt dimensions and the identical radiussed geometry of the flanged sleeves make possible an easy and effortless replacement of prior art plate packs by torque-optimized ones.